In a known type of optical pulse transmitter, a continuous-wave optical source is used to provide a continuous optical signal which is modulated in an electro-optical modulator by a NRZ electrical data signal to provide a NRZ modulated optical signal, and this NRZ optical signal is then converted to RZ form in a pulse transformer (essentially a second modulator) which is clocked at the same data rate as the data signal. Typically, the modulator used for the NRZ signal and the pulse transformer each take the form of a Mach-Zehnder modulator.
For such a pulse transmitter to work efficiently, it needs to be ensured that the bias applied to the two modulators is correct, so that they are both operated in the optimum portion of their operating characteristics, and the relative phase between the NRZ data signal and the RZ signal applied to the pulse transformer needs to be correct, so that the RZ pulses correspond to the widest-open part of the eye of the NRZ signal.
The problem of maintaining correct bias on the modulators is well understood and is satisfactorily solved by adding dither tone signals to various parts of the transmitter. A small part of the optical output signal of the transmitter is coupled out by an optical splitter and is detected by means of a photodetector which is slow enough not to respond to the pulse frequency, which is typically of the order of tens of GHz, 43 GHz being typical, but able to respond to the frequency of the dither tone signals, which is typically of the order of a few KHz, 2 KHz being typical. The electrical output signal from the photodetector is then demodulated to detect the amplitude at the frequency of the dither tone. The detected amplitude forms a control signal for a control loop.
Typically, the control loops for the two biases are not operated simultaneously, but in a time-multiplexed manner. That is to say, the dither tone is applied in turn to respective parts of the transmitter, and the detected amplitude used to control the two biases in turn. This makes it possible to have only one tone generator, one hardware filter at the tone frequency and the same demodulation processing for the two control loops. This technique is well known, works well and is commonly employed.
However, while this technique works well for controlling the bias, it does not lend itself to controlling the phase, for a number of reasons which will be explained below.